In optical networks, connections are set up and removed upon request of a customer. In existing networks, this is a manual and slow process, and as a result connections are usually established for long periods of time. In order to provide customers with bandwidth-on-demand, connection management will have to be carried out by automated means.
ITU Telecommunications Standardization Sector Study Group 13, “Requirements for the Automatic Switched Transport Network G.807/Y.1301 V1.0”, Caracas, May 14-25 2001, Ed. Michael Mayer, and ITU Telecommunications Standardization Sector Study Group 15, “Architecture for the Automatic Switched Optical Network (ASON) G.8080/Y.1304 V1.0”, Geneva, Oct. 15-26 2001 are a set of standards being developed for automatically switched optical networks (ASON). A transport network will be used to carry customer traffic. The transport network will contain interconnected network elements. The network elements will be connected by links of optical fibre entering the network elements at ports. An ASON control plane will provide connection management services to the transport network. The ASON control plane will include Optical Connection Controllers (OCCs) on which connection management software will run. Each OCC may be linked to one or more network elements through Connection Control Interfaces.
One aspect of connection management is bandwidth control. One problem in bandwidth control is load balancing. It is important that a port not be overused. While the OCCs should select a path for a connection having low metrics (such as cost or distance), the OCCs should also avoid selecting a path through a port that is near its maximum capacity. A simple threshold test can be used, but this may lead to a lot of signalling between the network element on which the port is located and an OCC as the amount of traffic through the port fluctuates around the threshold.
Another problem in bandwidth control is fragmentation. Fragmentation occurs when concatenated SONET payloads of different sizes are allocated and released on a SONET port. Each connection must be allocated to contiguous time slots. If the slot assignment and release is performed arbitrarily, some free slots may exist between assigned slots. In such a case, there may not exist sufficient free contiguous slots to accommodate a high capacity SONET connection request, even though there may be sufficient free slots in total, and the connection request will be rejected. For example, four STS-1 requests may have been assigned to slots 1, 49, 97, and 145 of an OC192 port (having 192 slots in total). Although the total number of free slots is 188, a new STS-48c request cannot be accommodated because there are no contiguous 48 free slots.
In addition, a standard may specify that certain concatenated payload types can only be assigned to certain slots. For example, an STS-12c connection can only start at slots 1, 13, 25, 37, etc. An STS-48c connection can only start at slots 1, 49, 97, or 145 (on an STS-192 port). For such payload types, a connection may be rejected even if there are sufficient contiguous free slots to accommodate the connection. For example, if STS-1 slots had been assigned to slots 2, 96, 97, and 145, an STS-48c connection cannot be accommodated despite the 93 contiguous slots from slot 3 to slot 95. The STS-48c connection would have to begin at slot 49, but would require slot 96 to be free.
Another aspect of connection management is fault notification and path restoration. When a link between two network elements fails, the ASON control plane must be notified of the fault. In particular, source nodes for connections interrupted by the fault must be notified, so that new paths can be determined to restore the connections, Under the currently proposed method of fault notification, the OCC responsible for a network element terminating the link determines the source nodes of the connections affected by the fault. The OCC then sends a control plane message to the source nodes using standard signalling (for example, using the Constraint based Routing Label Distribution Protocol), notifying the source nodes of the fault. While this results in a low amount of targeted fault notification, the determination of the source nodes takes time.
Once the ASON control plane is notified of a fault, a new path must be established to replace the path interrupted as a result of the fault.